1. Field of the Invention
The present invention relates generally to establishing communication links in communication systems, and particularly to establishing communication links between an inter-satellite link (ISL) antenna and a non-ISL antenna when both of the antennas are hosted on objects in space or intended to go into space.
2. Description of the Related Art
FIG. 1 illustrates a conventional system for transmitting information from a non-geostationary orbit (NGSO) satellite to either a cell phone or a commercial television satellite dish. NGSO satellite 1 transmits image data to ground station 2. Ground station 2 transmits the image data to the gateway 3 of a standard, commercial domestic satellite (DOMSAT) which does not possess inter-satellite link or inter-spacecraft link (ISL) antennas. DOMSAT gateway 3 then relays the image data through an up link to DOMSAT 4. DOMSAT 4 then transmits the data to either a satellite cell phone 7 or a commercial television satellite dish 8. To those skilled in the art, it is understood that formatting communications for the non-ISL antennas of a target satellite or DOMSAT is guided by the formatting standards of the communications device that an entity wants to reach through that target satellite or DOMSAT, especially if it is a bent-pipe satellite. When formatting a communications signal for the non-ISL antenna of target DOMSAT 4, one must format that communications signal to meet the standards of satellite cell phone 7 or commercial television satellite dish 8 in this example. The antennas on DOMSAT satellite 4 are non-ISL antennas because they are designed to transmit or receive image and other data from receivers located on Earth rather than transmitters and receivers located in space. Non-ISL antennas are not designed or optimized to conduct an inter-satellite or inter-spacecraft link. An antenna that is not optimized for inter-satellite or inter-spacecraft cross-links is defined as an antenna, on a spacecraft, that is not purposefully pointed towards the source or destination antenna that the non-ISL antenna intends to communicate with. When “pointing” or “tracking” is discussed, reference is being made to using both the antenna boresight and using the antenna main lobe of the non-ISL antenna to point or track towards the source or destination antenna the non-ISL antenna intends to communicate with. As defined by those skilled in the art, antenna boresight, also known as the axis of the antenna, is the direction of highest power density of the antenna, and the antenna main lobe includes within its pattern the antenna boresight.
FIG. 2 illustrates a system for transmitting information using a Tracking and Data Relay Satellite System (TDRSS) satellite. TDRSS is a communication relay system which provides inter-satellite and inter-spacecraft links (ISL) to relay communications between low earth orbiting (LEO) spacecraft and the ground. The antennas on a TDRSS satellite include ISL optimized antennas that point their antenna boresight at the source or destination antenna that they intend to communicate with. In FIG. 2, a space station 11, which is a LEO platform, uses an ISL antenna to establish an ISL link to a TDRSS satellite 13 and that TDRSS satellite's ISL antenna. Alternatively, a space shuttle 12, which is in LEO, establishes an ISL link with another one of the TDRSS satellite's ISL antennas. The ISL antennas on the TDRSS satellite then relay the data received from the space station or the space shuttle through a feeder or service link antenna aboard TDRSS down to a non-moving or low-relative-motion antenna at a ground station 14. Customer data is sent through ground station 14.
Besides the space station or a space shuttle, the NGSO satellite (FIG. 1) may be able to communicate information to a TDRSS satellite through an ISL link between an ISL antenna on the NGSO satellite and an ISL antenna on TDRSS.
Ground station 14 transmits the data received from the TDRSS satellite 13 to DOMSAT gateway 3 (in FIG. 1). DOMSAT gateway 3 then relays the data to DOMSAT 4. DOMSAT 4 then transmits the data through its non-ISL antennas to satellite cell phone 7 or commercial satellite television dish 8 or to another end user by relaying the data through DOMSAT 4. The two uplinks within this communication, one from the space station 11 to a TDRSS satellite 13, and the other from the DOMSAT ground station gateway 3 to the DOMSAT 4 are illustrative of what is known as a “double hop.”
ISL antennas are designed to move so as to track satellites which are communicating therewith, where the “boresight” of the antenna is steered to point at the satellite; thereby keeping the satellite within the largest gain portion of the ISL antenna, namely the boresight and main lobe of the ISL's antenna pattern, in communications range. ISL antennas are designed to communicate with another ISL antenna moving at over 17,000 mph relative to a stationary point on Earth. The single-access ISL antennas on TDRSS are designed to link with one custom-built LEO satellite ISL antenna at a time.
Non-ISL antennas on satellites are designed and used to communicate with aircraft or ground antennas within the Earth's atmosphere, and these non-ISL antennas are generally fixed and do not track satellites or other fast moving spacecraft with their antenna boresight. Some non-ISL antennas on satellites are pointable, but they are not designed to actively track an object outside of the Earth's atmosphere with their antenna boresight. They are designed to link with near-fixed antennas in the Earth's atmosphere. Compared to the 17,000 mph that satellites in space travel at, an aircraft's relative motion within the Earth's atmosphere appears near-fixed from the perspective of a satellite's non-ISL antenna.
In another conventional system, a Predator unmanned aircraft (not shown) transfers data through a 2-way communications link to a geostationary (GEO) satellite's non-ISL antenna. The link established by a Predator aircraft is not between two objects in space, and thus does not address the problems of establishing a link between two objects moving much faster relative to each other in space (i.e., around 17,000 mph), correcting for a much higher level of Doppler shift, tracking while traveling at such a high rate of speed, and pointing at a target at a much higher altitude with a more extreme off-antenna-boresight orientation.
The conventional design approach for an antenna used for satellite communications is to provide as much antenna gain as possible, while still being able to keep the target within the highest gain portion of the satellite's antenna pattern. The higher the antenna gain, the lower the required transmitter power needed to close a communication link between the satellite and the target. Usually high-gain antennas have characteristic “sidelobes,” which are lower gain regions of an antenna pattern. These sidelobes are typically not used for communicating with the intended target since the antenna gain in the sidelobes is lower than that of the main lobe of the antenna and much lower than the gain found when being on-axis with the antenna boresight.
The conventional design approach for an antenna used in inter-satellite crosslink communications is to point the antenna boresight and the antenna main lobe towards the source or destination antenna that that antenna is communication with. U.S. Pat. No. 5,579,536, the contents of which are hereby incorporated by reference in its entirety, states that an inter-satellite cross-link is maintained where both the source and destination or intermediate communication satellite's antennas are pointed toward each other and data communication is occurring. The conventional design approach for inter-satellite crosslink communications does not address the large pointing errors, the low gain, and the other problems of trying to conduct inter-satellite crosslink communications when at least one of the communicating antennas is not pointed towards or actively tracking the antenna it is trying to communicate with.
The above described satellite communication systems rely on dedicated communications equipment designed for single purposes. Such equipment is very expensive. For example, 30 million dollar ground antennas are common when relaying imagery or other data from a custom built satellite to the ground. When not using these expensive ground antennas, the alternative sometimes is to use an ISL data relay to a 500 million dollar or more expensive satellite with custom built ISL antennas on board.
“Aeronautical Broadband Communication Via Satellite,” by M. Werner and M. Holzbock, DLR Oberpfaffernhofen, Institute of Communications and Navigation describes an aircraft linking with non-ISL antennas on GEO satellites, and is hereby incorporated by reference in its entirety. However, this system does not function in space because it does not account for the additional speed a spacecraft travels at, the much higher altitude a satellite travels at, and the difficulties of maintaining communications when the communicating antennas spend more of their time in an off-boresight orientation.
U.S. Pat. No. 6,714,163, the contents of which are hereby incorporated by reference in its entirety, discloses a phased array aircraft antenna, which is not in space, accessing satellites.
U.S. Pat. No. 5,579,536, the contents of which are hereby incorporated by reference in its entirety, states that an inter-satellite crosslink is between two antennas that are pointed towards each other.
Other concepts relevant to satellite communications are found in U.S. Pat. Nos. 6,020,845, 6,775,251, 6,628,921, 5,825,325, 6,714,163, 6,603,957, and 5,812,538 the entire contents of which are incorporated herein by reference.
Also, the International Telecommunications Union (ITU) and the United States Federal Communications Commission (FCC) establish various requirements and regulations relevant to satellite communications, including regulations pertaining to pointing, power, frequency, and other requirements for inter-satellite links, the entire contents of which are incorporated by reference. In general, the ITU and FCC have separate and different regulations for the various requirements for inter-satellite crosslink communications when compared to their regulatory requirements for other satellite communications. This is to prevent these inter-satellite crosslink communications from interfering with other types of non-ISL satellite communications. The conventional ITU and FCC approach to separate the regulatory requirements for ISL communications and non-ISL communications does not address the opportunities or challenges associated with regulating an ISL communication to a non-ISL antenna or the various requirements of conducting ISL communications within the various regulatory requirements for non-ISL satellite communications. By preparing for the opportunity of ISL communications within non-ISL frequency and other regulatory requirements, the ITU or FCC could free a lot of additional frequency spectrum for increased uses.
However, as recognized by the present inventor there are multiple markets for an inter-satellite or inter-spacecraft communications system that can use the already-built communications infrastructure provided by current non-ISL satellite communication systems or that can mimic the performance and requirements of a non-ISL satellite communication system. For example, a spacecraft can acquire images of the Earth and transmit them directly to a television set by relaying these transmissions in only “one hop” through the non-ISL antenna of a television broadcasting DOMSAT in GEO and on to the targeted television set which is connected to a satellite TV antenna on the ground. A spacecraft can acquire data from outer space and relay this data in “one-hop” through the non-ISL antenna of a low earth orbit (LEO) Globalstar or Iridium satellite and then down to a satellite cell phone on the ground. A spacecraft on the Planet Mars can relay its data through a satellite in LEO, and then this same data can be relayed again through the non-ISL antenna of a DOMSAT in GEO and on to a cable television satellite head-end antenna on the ground. A satellite in LEO can receive a voice transmission from satellite cell phones or other ground stations on Earth and interface with the non-ISL antennas of an existing DOMSAT in GEO to relay that voice transmission to a satellite TV end-user through the audio or voice portion of their television set.
The conventional method of delivering images, voice, video, and data from a spacecraft to a ground antenna, and then later relaying that same information through a second satellite to a second ground antenna, requires the use of more communication infrastructure than relaying all of that same information directly from the original spacecraft through the second satellite to the second ground antenna. The additional communication infrastructure required for communicating with the conventional approach in this example would include, at minimum, an additional ground station 2 (from FIG. 1) and an additional DOMSAT gateway 3. Many satellite ground stations 2 and DOMSAT gateway antennas 3 are extremely expensive, because they are designed for specialized one-of-a-kind satellite communications requirements. Most satellite cell phones 7 and satellite TV antennas 8 are comparatively very inexpensive because they are designed to be produced in high quantities for a mass market of hundreds of thousands of end users. Replacing specialized one-of-a-kind satellite ground antennas with cheap mass consumer market satellite antennas could save tens of millions to hundreds of millions of dollars in satellite ground infrastructure construction and operation costs. In order to replace ground station 2 and DOMSAT gateway 3 antennas with satellite cell phones 7 or satellite TV antennas 8 in the example above, a spacecraft must be able to deliver images, voice, video, or data directly through DOMSAT's non-ISL antenna to satellite cell phones 7 or satellite TV antennas 8.
The conventional methods of delivering images, voice, video, and data through ISL communications systems to end users using non-ISL satellite communication systems suffers from a lack of hardware availability, because the hardware for conventional satellite communications systems is inflexible and cost prohibitive. Around 99 percent of the ground antennas that work with a satellite communications system only work with a satellite communications system that does not possess inter-satellite link antennas or other ISL hardware. The conventional approach for relaying ISL communications to these 99 percent of ground antennas requires the design, construction, and launch of extremely expensive custom-built satellites with a combination of specialized ISL and non-ISL antennas or the expensive “double hop” relay of ISL communications through an expensive ISL satellite with ISL antennas down to a ground station and then back up to another satellite with non-ISL antennas to relay to the ground again. Eliminating the design and use of ISL-specific satellites and “double hops” could save hundreds of millions of dollars in infrastructure and operational costs within a satellite communications system. In order to avoid these inefficient “double hops” or the design and use of custom-built relay satellites with ISL antennas, one must be able to use the existing non-ISL satellite communications infrastructure to conduct ISL communications.
Conventional ISL antennas are designed to communicate on-axis, on-antenna boresight, and within a narrow beam width and narrow antenna main lobe. There is currently a need for an alternative system to conduct inter-satellite cross-links with non-ISL antennas on spacecraft that were not designed for an inter-satellite cross link communications. There is currently a need for a method of conducting ISL cross-links off-axis and off-boresight with non-ISL antennas that do not point their antenna boresight towards fast-moving spacecraft. There is also a need for specialized apparatus which could improve the performance of these new and unconventional systems and methods. By designing such a system, method, and apparatus, one could avoid the expenses associated with the costly in-space and on-the-ground hardware of the conventional systems.
Current ISL equipment cannot relay communications through feeder or service (non-ISL) link antennas. Unique ISL missions, such as the Iridium satellite telephony system, create a need for expensive and custom designed ISL equipment that perform only ISL communications on each spacecraft.
Feeder or service (non-ISL) link space stations are designed for earth station communication standards. Non-ISL spacecraft antennas are not designed to track a spacecraft. ISL antennas and ISL spacecraft are designed to track and point at other spacecraft. ISL equipment is not designed to operate in conjunction with the frequency, modulation, regulatory, and other requirements for non-ISL antennas.
Therefore, what is desired is as discovered by the present inventor, is a communications method, a communications system, and a communications platform that can adapt to relay information from an ISL antenna in space to another satellite's non-ISL antenna.